Filiform Corrosion

Filiform corrosion is an attack of a metal substrate material underneath a polymeric film. The corrosion initiation is generally due to a defect in the coating. Corrosive elements to the metal substrate deposit in the defect area causing corrosion of the metal as well as bulging and cracking of the coating. The corrosion tends to spread one-dimensionally in a random manner creating patterns resembling a worm path or tentacles emanating form a point, see Figure 29. There is an associated “head” where the corrosion is preceding and the “tail” where the corrosion originated. Filiform corrosion has been observed on steel cans, aluminum foils and painted aluminum alloys, as well as other lacquered metals. It normally occurs in high humidity (> 65% RH) although it may result in lower humidity environments of severe corrosivity. The width of the corrosion paths are in the range of 0.05 – 3 mm depending on the coating material and the corrosivity of the environment.

Managing Filiform Corrosion

Methods that reduce filiform corrosion include the following:

• Use less active metal substrates
• Reduce humidity
• Use zinc primers on steel
• Use multiple coat/paint systems

Other corrosion protection methods can be used, and are discussed in terms of combating corrosion in general in Section 3.0.